In recent years, it has been become common to transmit not only audio data but also large-volume data such as still-image data and video data in cellular mobile communication systems in connection with the increasing development of multimedia information. Moreover, in LTE-Advanced (hereinafter referred to as “LTE-A”), techniques enabling a high transmission rate using a broad radio band, a MIMO (Multiple-Input Multiple-Output) transmission technique, and an interference control technique have actively been discussed in order to enable transmission of large-volume data.
Furthermore, various apparatuses are introduced as radio communication terminals in, for example, M2M (Machine to Machine) communication, or the multiplex number of a terminal increases with a MIMO technique, thereby causing anxiety about a shortage of resources in a region used for a control signal (PDCCH: Physical Downlink Control Channel). If a control signal cannot be mapped due to this shortage of resources, data cannot be assigned to the terminal. As a result, even if a data region used as a resource region for mapping data is available, the area cannot be used, so that a concern arises that the system throughput may decrease.
As a way of solving this problem, mapping a control signal toward a terminal served the base station to a data region has been discussed. The data region to which the base station maps control signals toward terminals served by the base station are called, for example, an E-PDCCH (Enhanced PDCCH) region, N-PDCCH (New-PDCCH) region, or X-PDCCH region.
Thus, mapping a control signal to a data region enables controlling the transmission power for a control signal to be transmitted to a terminal located near a cell edge, the interference given to other cells by a control signal to be transmitted or interference given to the cell provided by the base station from another cell.
However, PHICH (Physical HARQ Indicator Channel) used for uplink (UL) retransmission control transmitted in downlink (DL) indicates an ACK/NACK, but the number of bits used herein is small, so that transmission using a data region results in inefficiency.
Next, a UL retransmission control method of LTE will be described. In LTE, a DL grant (referred to also as DL assignment) serving as DL assignment control information indicating DL data assignment, and a UL grant serving as UL assignment control information indicating UL data assignment are transmitted on PDCCH. DL assignment is used to indicate that a resource in the subframe used for transmitting this DL assignment has been allocated to the terminal.
On the other hand, regarding a UL grant, in a FDD (Frequency Division Duplex) system, a UL grant is used to indicate that a resource in the target subframe which is the fourth subframe from the subframe used for transmitting this UL grant has been allocated to the terminal.
Moreover, in a TDD (Time Division Duplex) system, a UL grant is used to indicate that a resource in the target subframe which is the fourth subframe or a subframe after the fourth subframe from the subframe used for transmitting this UL grant has been allocated to the terminal. In the TDD system, after how many subframes from the subframe used for transmitting the UL grant, a subframe is allocated to the terminal as a UL subframe is determined according to a pattern of time-dividing UL and DL (hereinafter, “UL/DL configuration pattern”). However, in any UL/DL configuration pattern, the UL subframe is the fourth subframe or a subframe after the fourth subframe from the subframe used for transmitting the UL grant.
The UL retransmission control supports non-adaptive retransmission for assigning a retransmission signal to the same resource as the resource allocated last time and adaptive retransmission capable of assigning a retransmission signal to a resource different from the resource to which the signal is assigned last time (for example, refer to Non-Patent Literature (hereinafter, abbreviated as NPL) 1). In the non-adaptive retransmission, only a PHICH for transmitting an ACK/NACK signal is used as a retransmission control signal, and a NACK is transmitted through the PHICH to request retransmission. When retransmission is not requested, an ACK is transmitted through the PHICH. Retransmission can be indicated only through a PHICH in the non-adaptive retransmission, which in turn, provides an advantage of a low overhead for the control signal transmitted through DL needed in order to indicate retransmission.
Moreover, in the adaptive retransmission, an ACK is transmitted through a PHICH while retransmission and a resource for retransmission are indicated with a grant for indicating resource allocation information. A UL grant has a bit called an NDI (New Data Indicator), and this bit takes a binary value of 0 or 1. The terminal compares the NDI of a received current UL grant with the NDI of the last UL grant in the same retransmission process (HARQ (Hybrid ARQ) process). If there is a change in the NDI, the terminal judges that new data is assigned. If there is no change in the NDI, the terminal judges that retransmission data is assigned. In the adaptive retransmission, the resource amount and MCS (Modulation and Coding Scheme) can be changed according to a required SINR (Signal-to-Interference and Noise power Ratio) of the retransmission signal, which therefore provides an advantage of improving the frequency use efficiency.
A UL grant includes CRC (Cyclic Redundancy Check) and therefore the reliability of the received signal is high in comparison with a PHICH. Thus, when receiving a PHICH and a UL grant, the terminal follows an indication of the UL grant.
FIG. 1 illustrates a UL retransmission control procedure in a terminal. In FIG. 1, at Step (hereinafter abbreviated as “ST”) 11, whether a UL grant exists is determined. If a UL grant exists (YES), the process shifts to ST12. If a UL grant does not exist (NO), the process shifts to ST15.
In ST12, the NDI of the current UL grant is compared to the NDI of the last UL grant in the same retransmission process to determine whether the NDI changes. If the NDI changes (YES), the process shifts to ST13. If the NDI does not change (NO), the process shifts to ST14
In ST13, new data is transmitted to the base station. In ST14, retransmission data is adaptively retransmitted to the base station.
In ST15, whether a PHICH is NACK is determined. If the PHICH is NACK (YES), the process shifts to ST16. If the PHICH is not NACK (NO), the process shifts to ST17.
In ST16, retransmission data is non-adaptively retransmitted to the base station. In ST17 the process is shift to suspending, i.e., retransmission control is suspended.
Here, mapping of a PHICH will be explained. First, in encoding of a PHICH, ACK/NACK (1 bit) is subject to three times repetition. The number of PHICHs is one of {⅙, ½, 1, 2} times as many as the number of RBs and is indicated through a PBCH (Physical Broadcast Channel). By code multiplex with SF=4 and IQ multiplex, eight PHICHs can be transmitted with 3 REGs (=12 REs). Eight PHICHs placed at 3 REGs are called a PHICH group.
Mapping of a PHICH is dependent on a cell ID. Therefore, an interference control with other cells is difficult, and a PHICH interferes with PDCCHs of other cells and/or a CRS (Cell-specific Reference Signal); All 3 REGs included in a PHICH may be mapped to OFDM symbol #0, or the REGs may be respectively mapped to OFDM symbols #0, #1, and #2 as illustrated in FIG. 2. Information representing which mapping is used is indicated using broadcast information.
In LTE and LTE-A, each RB (Resource Block) consists of 12 subcarriers×0.5 msec, and the unit of combination of two RBs in the time domain is called an RB pair. Therefore, an RB pair consists of 12 subcarriers×1 msec. When representing a block of 12 subcarriers in the frequency domain, the RB pair may only be called an RB. Moreover, the unit of one subcarrier×one OFDM symbol is called one RE (Resource Element). Moreover, one REG (Resource Element Group) includes four REs.
Under the premise described above, NPL 1 proposes that, since the reception quality of a PHICH is inadequate in a terminal at a cell edge, adaptive retransmission without using a PHICH is set in a higher layer. When only adaptive retransmission supported without using a PHICH, retransmission is controlled only with a UL grant. Therefore, raising the aggregation level of the UL grant can improve the reception quality of a terminal at a cell edge.
Moreover, in NPL 2, the operation without using a PHICH is applied in a backhaul between a base station and a relay station. This is because a relay station transmits a control signal region to a terminal and thus does not receive a PHICH for the purpose of preventing interference due to coupling loop.
In this way, whether or not to use a PHICH is determined in units of terminals or systems in NPL 1 and NPL 2.